1. Technical Field
The invention concerns the field of recombinant gene engineering. It concerns novel introns and compositions comprising artificial introns as well as a method to improve expression of polypeptides from nucleic acids such as cloned genes, especially genes encoding antibodies and antibody-derived fragments, and the production of various polypeptides in eukaryotic host cells using said novel intron sequences.
2. Background
The market for biopharmaceuticals for use in human therapy continues to grow at a high rate with more than 300 biopharmaceuticals already approved, many more in clinical development and estimated sales of more than 167 billions by 2015. Currently, an increasing number of biopharmaceuticals is produced from mammalian cells due to their ability to correctly process and modify human proteins. Therefore the recombinant proteins are compatible with humans both functionally and pharmacokinetically. A shortcoming compared to prokaryotic expression systems is often the significantly lower protein expression level resulting in higher drug costs. Successful and high yield production of biopharmaceuticals from mammalian cells is thus crucial and is governed by various factors including host cell line, expression system, gene copy number, cell growth and productivity, secretion efficiency of the protein, culture and feed media, production and purification process, protein structure and sequence, protein stability and formulation. Expression of the recombinant protein requires an expression vector encoding the desired gene of interest. Several methods have been employed to optimize expression vectors for efficient protein production. Gene expression is regulated on transcriptional and translational levels. Hence many methods pertain to the identification and optimization of strong promoters and enhancers to improve the efficiency with which protein encoding genes are transcribed. Examples of these are the CMV immediate early promoter and enhancer, SV40 promoter and enhancer, elongation factor (EF) promoter, Polyoma enhancer, and chicken [beta]-actin promoter. Likewise, strong polyadenylation signal sequences such as bovine growth hormone (BGH) and SV40 polyadenylation sites that stabilize mRNAs and enhance transcription termination are also used to augment the protein expression from genes encoded by the expression vectors. Among the methods to improve the efficiency with which the resultant mRNA is translated are the use of translation initiation sites (AUG), optimal ribosome binding sites such as the Kozak sequence or internal ribosome entry sites (IRES) and the tripartite leader element (TPL) from adenovirus.
Another common approach to improve expression is to increase the gene copy number. This can be achieved by transfecting cells with selectable, amplifiable marker genes such as dihdrofolate reductase (DHFR) or glutamine synthetase (GS) genes and growing the cells in the presence of selective agents such as methotrexate in case of DHFR or methionine sulfoximine in case of GS.
By the chance integration of the expression vectors in the host cell genome, cells are obtained with different levels of expression of the desired gene product, as its expression is not determined solely by the strength of the transcriptional and translational regulatory elements described above. The chromatin structure present at the integration site can affect the level of expression both negatively and positively. Increasingly, therefore, cis-active elements which positively influence the expression at the chromatin level are integrated in expression vectors. These include locus control regions (LCR), scaffold/matrix attachment regions (S/MARs), ubiquitous chromatin opening elements (UCOE), expression augmenting sequence elements (EASE), transcription or expression enhancing elements (TE element) or stimulatory and anti-repressor elements (STAR).
Even though there exist prior art elements to increase the protein expression by modulating the expression vector, there is further need to identify regulatory elements to further increase the productivity of a recombinant production cell line.